[Code health] Eliminate global async states in favor of component @Inputs

Nested calls to subscribe() in `ngOnInit`, mixing signals + observables, and inconsistent and/or duplicate source-of-truth, and loose separation of concerns have made many of the components hard to grok and maintain. For example:

https://github.com/google/ground-platform/blob/a860c366f330afc9967271e889fb3579d13cefdf/web/src/app/pages/create-survey/create-survey.component.ts#L155-L195

Assigning this task to myself to come up with a plan to improve maintainability across the codebase.

Already done: #2301

@rfontanarosa FYI